1. Field of the Invention
The present invention relates to a magnetic recording and reproducing apparatus and, more particularly, to a slow-motion device for use in a magnetic recording and reproducing apparatus which is suitable for effecting noiseless slow-motion reproduction using a memory.
2. Description of the Prior Art
In the conventional helical can type magnetic recording and reproducing apparatus designed to record and reproduce an image signal on and from a magnetic tape in an amount which corresponds to one field in each read or write cycle, slow-motion reproduction is enabled by intermittently feeding the tape for each field of the recording pattern as described in Japanese patent laid-open No. 2018/1979 which corresponds to U.S. Pat. No. 4,190,869. However, in order to realize unblurred noiseless slow-motion reproduction with this type of prior art, it is, as is well known, necessary to enable high precision intermittent drive and employ a complicated head arrangement, an the control of the intermittent drive is considerably complicated. Further, intermittent feed of the tape causes variation of period in the horizontal and vertical synchronizing signals for reproduction through the repetitive stop and feed of the tape, resulting in the reproduced image being oscillated horizontally and vertically. Compensation for these oscillations also needs a complicated system, and it has heretofore been difficult to completely eliminate the oscillations of the reproduced image.
Thus, the prior art of the type described above suffers from the problem that, in order to realize unblurred noiseless slow-motion by reproducing the image signal for one field only in each read cycle, it is necessary to provide a complicated head arrangement, intermittent drive control means and a compensating system for eliminating horizontal and vertical swings of the reproduced image due to the variation of period in the horizontal and vertical synchronizing signals during the intermittent drive.
The above-described problems may be overcome by storing a memory with a steady image signal for one field during each stop of the intermittent drive and repeatedly reading the stored image signal during the feed of the tape in the intermittent drive.
In this case, the signal which determines the start point for reading and writing the image signal for one field must be synchronized with a point which corresponds to a blanking point that does not appear on the TV screen in order to obtain a continuous image, and the signal must also be synchronized with the reproduced image signal because the image signal is stored in the memory in synchronism with the intermittent drive.
Examples of the signals which satisfy the above-described requirements may be the vertical synchronizing signal reproduced from the magnetic tape and a signal for switching the video head for recording and reproducing the image signal such as that specified in the VHS standard for VTR.
However, these signals suffer from variation of period during the intermittent drive and cannot represent the length of one field from the point of time when the image signal has been stored in the memory. Therefore, if these signals are used, the reproduced image may be skewed or shifted vertically. Accordingly, when a memory is used for the above-described purposes, these problems must be taken into consideration.